When the drill string has actually parted or is stuck in the borehole, the operation for correcting the situation is called “fishing”. If the fish cannot be recovered, then it is cemented off and the borehole is sidetracked around it. Some of the tools used for fishing are described below.
• Mill: Milling is sometimes necessary in order to dress the top of a fish so that the selected fishing tool is able to make a firm positive catch. Mills usually are bladed or blunt, tungsten carbide coated, and are attached to the end of the drill string to be lowered into the borehole.
• Overshot: This is probably the first tool to be used when it is established that the top of the fish is relatively smooth. It will slide over the fish, center it, then use a rotary tap or slips to engage the fish.
• Wall-hook guide: This is used if the top of the fish is located in a washed out section. It takes the place of the regular guide on the bottom of an overshot. It will engage the fish and guide it into the overshot.
• Jar: This is used when the drill string is stuck or when a fish is caught in an overshot and cannot be pulled from the borehole. In normal drilling, the jar is placed in the heavy weight pipe section, while in fishing it is located directly above the fishing tool. Jarring provides a method of giving an upward jerk to free the pipe. It works similar to a trip-hammer.
• Free-point indicator and string shot: When jarring has not been successful, this is used to determine at what point in the borehole the fish is stuck. It is an electronic instrument that can sense torque or pipe movement. It is lowered on a wireline as far as possible and raised slowly while the drill string is stressed. Below the stuck point, no torque will be sensed. When the instrument gives a positive indication, the “free point” is reached.
The free point indicator is raised until string shot is positioned opposite the nearest tool joint (or one or two tool joints above the stuck point). Left-hand torque is applied to the drill string by the rotary table, and the primacord string shot is exploded. Loss of torque in the drill pipe is a definite indication that the tool joint has been loosened. The “back off” is completed by further left-hand rotation and by picking the pipe up a few feet.
• Washover: This is a large diameter pipe with a rotary cutting shoe on the bottom. It is used to “drill over” stuck pipe to free it before fishing.
• Spotting: This is used when jarring alone will not free the fish. Oil or special chemicals are spotted around the fish in an attempt to penetrate the wall cake, causing it to deteriorate and make the pipe slick. Spotting with water/oil when differentially stuck, and acid spotting when stuck in limestone are often used in an attempt to free the pipe.
• Safety joint: This is a coarse-threaded joint, which may be easily released. It is run above a fishing tool in case the fish cannot be freed and the fishing tool cannot be released. If spotting and jarring do not free the fish, the “free point” is used to locate the stuck point and the upper portion of the drill pipe is “backed off”. Fishing operations can then be carried out.
8. Wireline logging:
Downhole logs represent continuous measurements of the drilled formations as a function of depth. The advantage of downhole logging is the ability to record, concurrently, petrophysical as well as structural information of several properties. Operating on an intermediate scale between core measurements and borehole geophysics, downhole logs are characterized by fast data acquisition over large depth ranges under in situ conditions.
Most of the wells are logged by wireline systems. There usually two kinds of wireline logs: open hole logs and cased hole logs.
An electric logging company is called to the well while the crew trips out all the drill string. Using a laboratory, truck-mounted for land rigs and permanently mounted on offshore rigs the loggers lower devices called logging tools (or sonde's) into the well on wireline. The tools are lowered all the way to bottom and then reeled slowly back upwards. As the tools are coming up the hole, they are able to measure the properties of the formations they pass. Open hole logs
Open hole logging is done immediately after BHA is removed from the hole. There may be one, two, or more runs of the wire line logging. In normal and most common operations, a resistivity tool, gamma, and one or more porosity tools are used (such as neutron, density, or sonic tools). In exploratory wells, more sophisticated wire line tools are used.
Cased hole logs
Cased hole logging are run after casing is set and tools are used can work in the casing their signals can penetrate through the casing and into the formation. Examples of cased hole logs are cement bond logs, gamma logs for correlation, casing integrity test logs, etc.
9. Well completion:
The drilling of a well is only the first stage in the total life of that well. Following the drilling, the well must be “completed” in order to produce hydrocarbons at a commercial rate. When we take a close look at the drilling processes, we can understand why completions are so important.
As discussed before, when a well is drilled, the formation is first crushed by the drill bit, and then invaded by the drilling fluid. After drilling, the formation is surrounded by steel casing and weighted cement is pumped into the casing/formation annulus to bond the casing to the formation. After all of this, the target formation will need a little help if it is expected to produce hydrocarbons.
A typical completion consists of three components:
A wellhead assembly – the specialized surface equipment that seals and controls the wellbore.
A casing and tubing arrangement – provides improved control over the wellbore from the surface to the producing zone.
The bottom-hole or producing zone completion – improves control over the producing zone.
Wellhead:
A production wellhead is the assembly of specialized equipment that is located at the surface of a drilled wellbore, which seals the casing and tubing previously run into the well, permitting a controlled flow of produced fluids. This assembly of valves is commonly referred to as a “Christmas Tree”. The wellhead is installed during drilling operations and then modified as required if the well is to be produced. The wellhead consists of three basic components:
1. The casing head:
The casing head is a steel fitting (called a wellhead casing flange) that is connected to the top of the surface casing string at the wellhead. It supports the casing string until cemented into place.
2. The tubing head:
The tubing head is similar to the casing head but is smaller, and sits atop the casing head where it supports the tubing string.
3. The “christmas tree”:
The christmas tree is an assembly of fittings, valves, and chokes which control the rate of oil and gas flow from the well. It usually contains a pumping tee and gauge(s) and may contain a BOP preventer.
As indicated, a well completion will vary depending on the well and reservoir characteristics, as well as its economic potential. A variety of completion methods and procedures has been developed. Basically, completions can be divided into two categories: Single zone or multiple zones.
However, regardless of the type, the production casing or “oil string” must be set, the tubing arrangement determined, the packers must be properly placed and a decision must be made about the type of bottom-hole completion that will be used on the well.